1. Field of the Invention
The present invention relates to a film which can easily be torn by hand, generating no shavings and producing a roll of film which is reduced in volume. The present invention also relates to a process for producing the same. A film which can easily be torn by hand is a film having good "hand-tearability". The hand-tearability is a property allowing the film to be torn by hand without using any special cutting tools such as scissors or cutters. The representative examples of such a film invented are: a pressure-sensitive adhesive tape, a pressure-sensitive adhesive film, a substrate thereof and a film for wrapping fresh food such as vegetables.
2. Description of the Related Art
A thin material such as a film is often dealt with in a form of a roll in its production, storage and transportation. The size, such as width and thickness, of the roll is set depending upon its usage and the like. "A roll of film having reduced volume" means that thickness of the roll of the film of predetermined length is less than that of conventional films of the same length.
A film which has a lot of fine perforations opening through its thickness and can be used as a substrate of a pressure-sensitive adhesive tape is disclosed in the Japanese unexamined patent publication Hei 8-199123. This film is made of plastics such as polyester and polypropylene and has tapered perforations.
The above Japanese publication discloses a process for producing a film wherein an unprocessed film is pressed between a first roller to whose surface fine diamond particles are attached and a second roller having a hard smooth surface and perforations are opened in the unprocessed film by the fine particles.
However, both the roll of film disclosed in the above publication and the roll of a pressure-sensitive adhesive tape having that film which is used as a substrate are problematic in their volume. It is reasonable to assume that the reason of that problem is as follows:
The perforations are opened by a physical method in which an object having a sharp tip such as a diamond particle is pressed onto the unprocessed film against its first (front) surface toward its second (back) surface opposite to the front surface between the rollers. Burrs which occurred at the formation of the perforations protrude on the back surface of the film. As a result, the roll of the film with its protruding burrs increases in volume due to the presence of the burrs.
When a film having protruding burrs is used as the substrate for a pressure-sensitive adhesive tape and a pressure-sensitive adhesive is applied to its front surface on which the burrs are not present, a roll of the pressure-sensitive adhesive tape obtained becomes thick due to its still remaining protruding burrs and, thus a volume problem occurs. On the other hand, when the pressure-sensitive adhesive is applied to the back surface, on which the burrs are, the entire burrs must then be covered with a pressure-sensitive adhesive in order to achieve satisfactory pressure-sensitive adhesion, and as a result, the pressure-sensitive adhesive tape becomes thick and thus also a volume problem occurs.
Moreover, in the employment of the film having the protruding burrs as a wrapping material for wrapping fresh food, when the film is used with its back surface on which the burrs protrude as the inner surface of the wrapping material, shavings of the burrs occur by its rubbing against itself and that with the food. Additionally, some effort to recognize the front and back side of the film is required before using the film.